T. G. Prasad

Using a one‐dimensional turbulent closure model, we investigate the physical mechanisms governing the seasonal evolution of mixed‐layer depth (MLD) and sea surface temperature (SST) along two meridional sections; 64.5°E and 88.5°E, which are representative of the open‐ocean conditions of the Arabian Sea and the Bay of Bengal. Several perturbation experiments have been performed to isolate and judge exactly the relative importance of wind stress and buoyancy forcing to the evolution of MLD and SST. The wind stress forcing is found to be significant during the summer monsoon, while buoyancy flux plays a significant role during the winter monsoon. During the summer monsoon (June–September), the MLD in the Arabian Sea deepens to 80 m owing to strong wind‐driven vertical mixing, but the weaker winds over the Bay of Bengal restrict the turbulent mixing to a shallow depth of ∼30 m. As a result, SST in the Arabian Sea (Bay of Bengal) decreases to ∼27°C (remains high ∼29°C). It is thus the asymmetry in the wind field that is responsible for the difference in MLD and SST between the Arabian Sea and the Bay of Bengal. During the winter monsoon (November–February), the MLD in the Arabian Sea penetrates to deeper than 80 m by convective vertical mixing because of strong negative buoyancy flux (net heat loss), but it is restricted to 40–50 m in the Bay of Bengal owing to reduced net heat loss from the ocean. It is the asymmetry in the buoyancy forcing caused by the difference in latent heat flux that is responsible for the difference in MLD and SST between the Arabian Sea and the Bay of Bengal. The atmospheric parameter that controls this buoyancy difference is the difference in humidity between the two basins. Thus it is the differences in surface forcing, rather than differences in the vertical salinity stratification, that are key in explaining the observed and modeled differences between the Arabian Sea and Bay of Bengal.

During a typical Indian Ocean Dipole mode (IOD) event the weakening and reversal of winds in the central equatorial Indian Ocean lead to the development of unusually warm sea surface temperatures (SSTs) in the western Indian Ocean. Here we report an analysis of model results and observations for the 1990–1999 period, showing the anomalous warming that occurred during 1997–1998 and 1994–1995 IOD events in the western Indian Ocean. It was primarily the result of the internal oceanic processes within the Arabian Sea rather than a direct response to external forcing from the eastern Indian Ocean as previously suggested. We compare the evolution of subsurface temperatures in the Arabian Sea just prior to (1996–1997: Representative of a normal year) and during the IOD event (1997–1998) to infer the physical processes responsible for the anomalous warming during the IOD events. In a normal year such as 1996 the existence of a unique open ocean upwelling feature, the Arabian Sea Dome (ASD), raises the thermocline in the southern Arabian Sea, a result of the positive wind stress curl of the winter monsoon. During an IOD year (1997), with the collapse of the monsoon wind system in the Arabian Sea the cold tongue, the strip of cool water indicative of ASD that normally occupies the southern Arabian Sea, fails to develop. As a result, the thermocline remains deep, which in turn enhances the heat content of the upper ocean, leading to warm SSTs. Volume‐integrated heat budget terms in the ASD region illustrate the role of horizontal and vertical advection in the upper ocean warming. An implication is that SSTs remain higher than 28°C in the southern Arabian Sea, thereby supporting large‐scale deep convection in the atmosphere during the winter monsoon. A band of deep convective cloud forms directly over the warm ASD region during the IOD events, which bring greater than average rainfall over the southern Arabian Sea and east Africa, whereas convection is suppressed over the cold ASD region during the normal years resulting in less than average rainfall.